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I connected the V- pin to ground and the V+ pin to +5V. I then connected in+ to ground and in- to +5V. I expected Vout to be a low value near ground (0V) but I'm getting about +1.7V. Is this normal?

Steven Lu
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    When running a +/- op-amp on a single supply voltage it is usual to bias the in+ to somewhere in-between ground and V+ (say at 50% with 2 matching resistors in a voltage divider). This provides a 'fake' ground signal. V+ becomes +½V and ground becomes -½V. – Majenko Jun 18 '11 at 13:56
  • @Matt, thanks that does make sense. But I'm trying to build a voltage controlled current source and these limitations seem to mean that I'll need a ton of other components to hack it into a workable circuit. Time to order new parts methinks. – Steven Lu Jun 18 '11 at 14:15
  • @Steven if you don't mind the delay you could get some free samples from National, Analog Devices, Maxim, etc... – Majenko Jun 18 '11 at 14:30
  • @Matt - I think Steven first wants to know *what* to order (either as sample or purchase) :-) – stevenvh Jun 18 '11 at 14:37
  • @stevenvh which is where your answer comes in. These comments are not just for the benefit of the OP but for others who come after. – Majenko Jun 18 '11 at 14:43
  • The process goes faster if I can get my components faster. The decisions I have to make are which components to get, and usually whether I want to order just one, or 25 of them. – Steven Lu Jun 18 '11 at 14:44
  • Also I did not know you could get free samples. Which are always good. :) – Steven Lu Jun 18 '11 at 14:59
  • @Steven I run my whole electronic life on free samples and salvaged components - I rarely buy anything for tinkering with. – Majenko Jun 18 '11 at 16:08
  • thats awesome. how long does it take to get free samples after requesting them? – Steven Lu Jun 18 '11 at 17:34

1 Answers1

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Standard opamps can't work with input voltages close to the power supply, neither V+ or V-. Same for the output: they don't go all the way to V+ or V-. What you need is a rail-to-rail I/O opamp. (There are also opamps which are only input or output rail-to-rail!).
Also, most opamps want a dual supply, often minimum +5V/-5V. Since you're using it as a single supply opamp, your V+ - V- difference may be too low for normal operation.
Look for a single supply opamp. They often work at low power supply voltages only, and most of them will be rail-to-rail I/O.

note: actually, any dual supply opamp can be used as a single supply; since voltage is relative the opamp doesn't know whether V- is a negative voltage or ground. By single supply opamp I mean specifically an opamp which is low voltage, like I mentioned above.

stevenvh
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  • Does the LM324DR (which is a quad) fulfill this role? I'd like it to be able to drive Vout down to ground while using it with a single supply (so it will work with PWM). – Steven Lu Jun 18 '11 at 14:12
  • @Steven - the LM324 is a classic, a bit like \$\mu\$A741. It works from 3V (good) but isn't rail-to-rail (bad). [This overview](http://www.national.com/mpf/LM/LM324.html#Overview) says "Large output voltage swing 0V to V+ - 1.5V". In the datasheet look for "output voltage swing". – stevenvh Jun 18 '11 at 14:21
  • @Steven - you really have to look for **rail-to-rail** – stevenvh Jun 18 '11 at 14:22
  • do you know of one off the top of your head which is a rail-to-rail and a bit of a classic? I'd like to stick with component choices that aren't too exotic but it isn't easy to tell at a site like digikey. Also, "push-pull rail-to-rail"? "differential rail-to-rail"? – Steven Lu Jun 18 '11 at 14:45
  • I think "single supply" is op-amp code for "goes to the negative rail but not the positive rail". Fortunately, so many modern systems are battery-powered that RRIO op amps are quite easy to find. – markrages Jun 18 '11 at 14:46
  • @Steven - [this page](http://www.national.com/cat/index.cgi?i=i//11) lists a few dozen rail-to-rail opamps. You can filter further for single devices, for example. – stevenvh Jun 18 '11 at 14:49
  • how about MCP6004? thanks for the link. What does "VCM Min/Max from Neg/Pos Supply" mean? – Steven Lu Jun 18 '11 at 14:58
  • @Steven - the MCP6001 (single version, the 6004 has 4 opamps) looks good. Is not fast, but that [may not be important anymore](http://electronics.stackexchange.com/questions/15325/controlling-high-output-leds-with-a-microcontroller/15605#15605) – stevenvh Jun 18 '11 at 15:24
  • [TLV274](http://focus.ti.com/lit/ds/symlink/tlv274.pdf) is a good rail-to-rail output part that I often use on a single-ended +12V rail. Common-mode input range is a little better than LM324 (VDD-1.35) so it may not suit your needs 100%. – Adam Lawrence Jun 18 '11 at 16:31
  • TLV274 looks like the ticket. I'll pick up a couple MCP6004's too. thanks madman. also much thanks to stevenvh for all the handholding; i appreciate your patience! – Steven Lu Jun 18 '11 at 17:49
  • @Steven - you're welcome. Hope you've got enough info and ideas to finish your project. Otherwise just ask. Success! – stevenvh Jun 19 '11 at 08:24
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    I just did some tests. The MCP6004 works great, and is amazing value at 50 cents for one IC which contains four opamps. I do not have an oscilloscope though, so I can't test their response time. I'm intending to use these op amps to current-control LEDs, I imagine that if they have a response time under 1ms I will be okay. Now that I have put it in those terms I'd be surprised if it is slower than that. – Steven Lu Jun 22 '11 at 18:38